This invention relates to a semiconductor laser module, and, more particularly, to a semiconductor laser module for optical communications over a wide range of working temperature.
A semiconductor laser module for optical communications is proposed in JP-A-Hei- 3-155509 which, as shown in FIG. 1, comprises an optical device including an optical transmitter or detector mounted in a package 1, and an optical fiber 3 having one end face end faces located at an image-forming position 2a of a lens 2 and protected by a cylindrical ferrule fixed to the package 1 through a ring fitted over the lens 2 disposed in front of the optical device, wherein the end face of said optical fiber 3, directed to the optical device, is arranged so as to be substantially in the same plane as a joint plane of the lens-engaging ring and the package 1, and wherein a lens holder 4, made of a metal rod for holding the lens 2, is a monolithic body formed by a single material. In FIG. 1, reference numeral 5 denotes a heat sink and reference numeral 6 denotes a semiconductor laser.
As mentioned above, the lens holding member constituted by the lens holder 4 for holding the lens 2 and mounted in the package 1 as a component part of the semiconductor laser module is monolithic and made by a material of one kind.
However, since the material for the package main body 1, Kovar (KV), nickel alloy, etc. have been used to form air-tight terminals, consideration of a thermal expansion coefficient relationship with a glass or ceramic as an air-tight sealing material must be given consideration.
In the prior art, relative to the package 1, it is necessary to mechanically stably fix the lens holder member 4, made of a metal rod, for holding the lens 2 when the lens holder is connected to the package 1. Therefore, for the lens holder member 4, a material with a thermal expansion coefficient equivalent to the material of the package is used. So, a difference arises between the thermal expansion coefficient of the lens holder 4 and that of the lens 2. In the above-mentioned prior art, however, because a solder with a melting point of 200.degree. C. or below is used as a fixing material for fixing the lens 2, the above problem attributable to a difference in the thermal expansion coefficient has not occurred.
Yet, in the prior art, if an attempt is made to fix the lens 2 with joining means having a high melting point of 280.degree. C. or higher, cracks occur in the lens 2 caused by thermal stress; therefore, it is impossible to fix the lens with high-melting-point joining means.
In recent years, the working temperature range required of the semiconductor laser modules for optical communications has been expanded to a wider range of -40.degree. C. to +85.degree. C. instead of the conventional range of 0.degree. C. to +65.degree. C.
With the above-mentioned prior art, it is impossible to fix the lens 2 with high-melting-point joining means, which assures high reliability in the required range of working temperature, and thus, the problem with the prior art is that it does not have sufficient reliability against changes in the thermal environment.